FIG. 2 is a schematic showing a typical subsea station 108 with a bypass header and isolation valves 232 and 234. The pressure inside the subsea station 108 when isolated from the production flow line will in most cases deviate from both the ambient sea pressure and from the pressure in the production flow line. The pressure difference can often be significant, especially at large water depths and high design pressure systems. This pressure difference needs to be equalized in a controlled manner in order to prevent damaging equipment due to rapid pressure changes.
A current method for pressure equalization is to install two or more valves 202 and 204 in series with a small volume between the valves. The pressure can be equalized in small steps by first opening and then closing the valve closest to the high-pressure side. Then opening and closing the valve closest to the low-pressure side. The pressure can in this manner by repeating the process several times be fully equalized between the high- and low-pressure sides.
The above-described method will cause a series of instantaneous pressure changes which magnitude will depend on the size of the volume between the two valves. A valve can usually only be operated a certain number of times before it starts leaking/malfunctioning. Hence, there is also a lifetime limitation regarding how often a valve can be opened and closed before the valve will malfunction. Traditional nozzle designs should not be used to slow down the pressure change due to high velocities, cavitation danger, unknown/uncertain behaviour at low Reynold numbers (transient and laminar zone) etc.